1. Field of the Invention
The present invention relates to a load driving circuit, in particular, a circuit for driving an inductive load.
2. Description of Related Art
An inductive load such as a motor or a solenoid used in a field of electrical equipment for automobiles or other fields is usually connected to a circuit board via connectors and wiring lines. If a short circuit occurs in such a load as described above, an excessive current may flow in a switching device such as a transistor for controlling the load. If the excessive current continues to flow in the switching device, the switching device may be broken down.
Therefore, a protection circuit is usually provided for turning off the switching device to cut off the excessive current when a short circuit occurs in the load. For instance, Japanese Patent Application Laid-open No. Hei 7-87771 (hereinafter referred to as Sawano) discloses a motor speed and position estimating device having a function of prohibiting motor drive when an excessive current flowing in the motor as a load is detected. This motor speed and position estimating device includes four switching devices that are connected in an H-bridge and are driven by a pulse width modulation method (PWM method), a motor connected in a middle portion of the H-bridge, and a shunt resistor that is connected in series to the switching device of the H-bridge. A motor current is detected from a voltage across the shunt resistor. If the motor current is an excessive current, the drive of the motor is prohibited.
The analysis described below is given in the present invention.
In the device of Sawano, a current sensing resistor (shunt resistor) is inserted in series to the motor as a load, and an excessive current is detected based on a voltage drop across the current sensing resistor. In this structure, the connection of the current sensing resistor in series to the load causes deterioration of operation efficiency of the load. For instance, it is supposed that the motor has an internal resistance of 70 milliohms, each metal-oxide semiconductor field-effect transistor (MOSFET) as the switching device has an ON resistance of 10 milliohms, the current sensing resistor has a resistance of 10 milliohms, and a power supply voltage (VDD) is 10 volts. Then, a voltage of approximately 7.8 volts may be applied across the motor without the current sensing resistor, but a voltage of only 7 volts may be applied across the motor with the current sensing resistor. This may cause a drop in motor rotation speed (depending on the applied voltage) and torque (depending on motor current). In order to improve the motor rotation speed and the torque, it is necessary to decrease the resistance of the current sensing resistor extremely. In this case, a size of the resistor element having a low resistance will be large, which may cause an increase in area of a circuit board. In addition, it may also cause an increase in cost thereof.